Weathering & Soil Formation Notes
1. Soil is a mixture of rock particles, minerals, decayed organic material, water,
and air.
2. Soil particles from largest to smallest: gravel, sand, silt, clay
3. How does soil form?
a. Step 1: The C horizon forms as bedrock weathers & rock breaks up into
soil particles.
b. Step 2: The A horizon develops as plants add organic material to the soil
& plant roots weather pieces of rock (living organisms – chemical
weathering)
c. Step 3: the B horizon develops as rainwater washes clay & minerals
from the A horizon to the B horizon
4. Four soil decomposers & how they affect the soil:
a. Fungi (molds, mushrooms) – grow on & digest plant remains
b. Bacteria (microscopic decomposers) – cause decay by attacking dead
organisms
c. Very small animals (mites & worms) –decompose organic material &
mix it with soil
d. Earthworms – break up the soil, allowing in air & water
5. Earthworms are most important in mixing humus into the soil as they eat
their way through the soil, they carry humus down to the subsoil & subsoil
up to the surface. They also pass out the soil they eat as waste which is
enriched with substances that plants need to grow.
6. If all decomposers were removed from the soil the fertility of the soil would
decrease because the roots of the plants would not get air, water could not
move through as easily without the holes they can make, nutrients would not
be mixed, and much of the organic material would not be added to the soil.